A Lewis-acid monopolar polymer with both hydrophobicity and lipophilicity is used as the gate dielectric for all-inkjet-printed organic thin-film transistors. The hydrophobicity of this polymer prevents water molecules from migrating and being trapped in the gate dielectric, while its lipophilicity allows good wetting by organic solvents for further deposition of other functional layers. These organic thin-film transistors demonstrate high bias-stress stability under both positive and negative bias in ambient air. This study unlocks the potential of all-inkjet-printed organic thin-film transistors for real-world low-cost large-area applications.